(1) Field of the Invention
The present invention is related to an optical modulator, in particular an optical modulator used as an optical intensity modulator, phase modulator, and polarized wave modulator, for high-speed and high-capacity optical fiber communications.
(2) Related Art Statement
Recently, with the progress of high-speed and high-capacity optical fiber communications, as represented by the exterior modulator, high-speed modulators using a substrate formed of a material having electro-optical effect, like lithium niobate, are in practical use. This high-speed modulator comprises substrate 1 formed of a material having electro-optical effect, with waveguide 2 that guides the optical wave, and modulate electrode comprising ground electrode 4 and signal electrode 3 that applies the high-speed modulate signal in the microwave band to the said optical wave, as shown in FIG. 1.
To optical waveguide 2, an optical wave will enter from the end side of optically polished substrate. When passing the optical waveguide 2, the phase of the optical wave will transform because of changing reflective index of the substrate by the electric signal applied to the electrode, and so, in the Mach-Zehnder optical modulator like FIG. 1, phase transformation will be the strength modulation of the optic. Then, the optical wave, which has been affected with the strength transformation according to the electric signal, will exit from the other end of the optical waveguide 2.
The electric connection of the signal electrode 3 and ground electrode 4 of the optical modulator is usually done from the side of the substrate because of the convenience of the wiring usage, and also electric signal connection pad 6 is located on the signal electrode 3 to connect the wiring from the coaxial cable.
Microwave, which is an electric signal, is provided from the coaxial cable, is conveyed to the signal electrode 3 through the electric signal connection pad, goes thorough the bend point 7 of the signal electrode 3, and goes into the interaction part 8 of the optical wave-guide 2. In these wirings, because of the rapid change of the electric property of electrical line, part of the micro wave as an electric signal reflects at the connecting point, other parts escape into the substrate, and other left parts radiate outside the substrate in consequence. For this reason, electric signal which transmits to the interaction part 8 of the signal electrode 3 decreases, and effective modulation of the optical wave which passes through the optical wave guide becomes difficult.
To counteract this problem, efforts to reduce the loss of microwaves' reflection and escape, by forming the signal electrode 3 and ground electrode 4 to be as a co-planar type of plane electrode, connecting coaxial cable by impedance matching, or easing the bend point of the signal electrode 3, have been tried, but effective reduction was difficult in the high frequency zone over 20 GHz.